Subject: Re: [PATCH bpf-next v5 02/13] xsk: Add TX timestamp and TX checksum offload support

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 20 Nov 2023 12:45:00 -0800

On 11/16, Jesper Dangaard Brouer wrote:
> 
> 
> On 11/15/23 14:37, Stanislav Fomichev wrote:
> > On 11/15, Jesper Dangaard Brouer wrote:
> > > 
> > > 
> > > On 11/13/23 18:02, Stanislav Fomichev wrote:
> > > > On 11/13, Jesper Dangaard Brouer wrote:
> > > > > 
> > > > > 
> > > > > On 11/13/23 15:10, Stanislav Fomichev wrote:
> > > > > > On Mon, Nov 13, 2023 at 5:16 AM Jesper Dangaard Brouer <[email-masked]> wrote:
> > > > > > > 
> > > > > > > 
> > > > > > > On 11/2/23 23:58, Stanislav Fomichev wrote:
> > > > > > > > diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
> > > > > > > > index 2ecf79282c26..b0ee7ad19b51 100644
> > > > > > > > --- a/include/uapi/linux/if_xdp.h
> > > > > > > > +++ b/include/uapi/linux/if_xdp.h
> > > > > > > > @@ -106,6 +106,41 @@ struct xdp_options {
> > > > > > > >      #define XSK_UNALIGNED_BUF_ADDR_MASK \
> > > > > > > >          ((1ULL << XSK_UNALIGNED_BUF_OFFSET_SHIFT) - 1)
> > > > > > > > 
> > > > > > > > +/* Request transmit timestamp. Upon completion, put it into tx_timestamp
> > > > > > > > + * field of struct xsk_tx_metadata.
> > > > > > > > + */
> > > > > > > > +#define XDP_TXMD_FLAGS_TIMESTAMP             (1 << 0)
> > > > > > > > +
> > > > > > > > +/* Request transmit checksum offload. Checksum start position and offset
> > > > > > > > + * are communicated via csum_start and csum_offset fields of struct
> > > > > > > > + * xsk_tx_metadata.
> > > > > > > > + */
> > > > > > > > +#define XDP_TXMD_FLAGS_CHECKSUM                      (1 << 1)
> > > > > > > > +
> > > > > > > > +/* AF_XDP offloads request. 'request' union member is consumed by the driver
> > > > > > > > + * when the packet is being transmitted. 'completion' union member is
> > > > > > > > + * filled by the driver when the transmit completion arrives.
> > > > > > > > + */
> > > > > > > > +struct xsk_tx_metadata {
> > > > > > > > +     union {
> > > > > > > > +             struct {
> > > > > > > > +                     __u32 flags;
> > > > > > > > +
> > > > > > > > +                     /* XDP_TXMD_FLAGS_CHECKSUM */
> > > > > > > > +
> > > > > > > > +                     /* Offset from desc->addr where checksumming should start. */
> > > > > > > > +                     __u16 csum_start;
> > > > > > > > +                     /* Offset from csum_start where checksum should be stored. */
> > > > > > > > +                     __u16 csum_offset;
> > > > > > > > +             } request;
> > > > > > > > +
> > > > > > > > +             struct {
> > > > > > > > +                     /* XDP_TXMD_FLAGS_TIMESTAMP */
> > > > > > > > +                     __u64 tx_timestamp;
> > > > > > > > +             } completion;
> > > > > > > > +     };
> > > > > > > > +};
> > > > > > > 
> > > > > > > This looks wrong to me. It looks like member @flags is not avail at
> > > > > > > completion time.  At completion time, I assume we also want to know if
> > > > > > > someone requested to get the timestamp for this packet (else we could
> > > > > > > read garbage).
> > > > > > 
> > > > > > I've moved the parts that are preserved across tx and tx completion
> > > > > > into xsk_tx_metadata_compl.
> > > > > > This is to address Magnus/Maciej feedback where userspace might race
> > > > > > with the kernel.
> > > > > > See: https://lore.kernel.org/bpf/ZNoJenzKXW5QSR3E@boxer/
> > > > > > 
> > > > > 
> > > > > Does this mean that every driver have to extend their TX-desc ring with
> > > > > sizeof(struct xsk_tx_metadata_compl)?
> > > > > Won't this affect the performance of this V5?
> > > > 
> > > > Yes, but it doesn't have to be a descriptor. Might be some internal
> > > > driver completion queue (as in the case of mlx5). And definitely does
> > > > affect performance :-( (see all the static branches to disable it)
> > > > >    $ pahole -C xsk_tx_metadata_compl
> > > > > ./drivers/net/ethernet/stmicro/stmmac/stmmac.ko
> > > > >    struct xsk_tx_metadata_compl {
> > > > > 	__u64 *              tx_timestamp;         /*     0     8 */
> > > > > 
> > > > > 	/* size: 8, cachelines: 1, members: 1 */
> > > > > 	/* last cacheline: 8 bytes */
> > > > >    };
> > > > > 
> > > > > Guess, I must be misunderstanding, as I was expecting to see the @flags
> > > > > member being preserved across, as I get the race there.
> > > > > 
> > > > > Looking at stmmac driver, it does look like this xsk_tx_metadata_compl
> > > > > is part of the TX-ring for completion (tx_skbuff_dma) and the
> > > > > tx_timestamp data is getting stored here.  How is userspace AF_XDP
> > > > > application getting access to the tx_timestamp data?
> > > > > I though this was suppose to get stored in metadata data area (umem)?
> > > > > 
> > > > > Also looking at the code, the kernel would not have a "crash" race on
> > > > > the flags member (if we preserve in struct), because the code checks the
> > > > > driver HW-TS config-state + TX-descriptor for the availability of a
> > > > > HW-TS in the descriptor.
> > > > 
> > > > xsk_tx_metadata_compl stores a pointer to the completion timestamp
> > > > in the umem, so everything still arrives via the metadata area.
> > > > 
> > > > We want to make sure the flags are not changing across tx and tx completion.
> > > > Instead of saving the flags, we just use that xsk_tx_metadata_compl to
> > > > signal to the completion that "I know that I've requested the tx
> > > > completion timestamp, please put it at this address in umem".
> > > > 
> > > > I store the pointer instead of flags to avoid doing pointer math again
> > > > at completion. But it's an implementation detail and somewhat abstracted
> > > > from the drivers (besides the fact that it's probably has to fit in 8
> > > > bytes).
> > > 
> > > I see it now (what I missed). At TX time you are storing a pointer where
> > > to (later) write the TS at completion time.  It just seems overkill to
> > > store 8 byte (pointer) to signal (via NULL) if the HWTS was requested.
> > > Space in the drivers TX-ring is performance critical, and I think driver
> > > developers would prefer to find a bit to indicate HWTS requested.
> > > 
> > > If HWTS was *NOT* requested, then the metadata area will not be updated
> > > (right, correct?). Then memory area is basically garbage that survived.
> > > How does the AF_XDP application know this packet contains a HWTS or not?
> > > 
> > >  From an UAPI PoV wouldn't it be easier to use (and extend) via keeping
> > > the @flags member (in struct xsk_tx_metadata), but (as you already do)
> > > not let kernel checks depend on it (to avoid the races).
> > 
> > I was assuming the userspace can keep this signal out of band or use
> > the same idea as suggested with padding struct xsk_tx_metadata to keep
> > some data around. But I see your point, it might be convenient to
> > keep the original flags around during completion on the uapi side.
> > 
> > I think I can just move flags from the request union member to the outer
> > struct. So the struct xsk_tx_metadata would look like:
> > 
> > struct xsk_tx_metadata {
> > 	__u32 flags; /* maybe can even make this u64? */
> > 
> 
> Yes to u64 for two reasons (1) this becomes UAPI and
> (2) better alignment for tx_timestamp.
> But I'm open to keeping it u32.
> 
> > 	union {
> > 		__u16 csum_start;
> > 		__u16 csum_offset;
> > 	} request;
> > 
> > 	union {
> > 		__u64 tx_timestamp;
> > 	} completion;
> > 
> > 	__u32 padding; /* to drop this padding */
> > };
> > 
> > But I'd also keep the existing xsk_tx_metadata_compl to carry the
> > pointer+signal around. As I mentioned previously, it's completely
> > opaque to the driver and we can change the internals in the future.
> > 
> 
> Sure, it is an implementation detail and my objections are mostly that I
> don't find it as a pretty code approach that can be hard to follow.
> Maybe driver developer will object and change this later if it cost too
> much to increase the element size in their TX-ring queues.

To make sure I understand, your preference is to save the flags, right?
A potential problem with that approach might be that we'd also have to
carry the pointer to the original umem chunk (blowing the overhead by extra
8 bytes) or pulling it of the tx completion descriptors in the drivers (extra
complexity). Pulling it out of the tx completion also might be
problematic because because we store iova/dma addresses in the
descriptors?